Lesson 2

 

Volume Measurements

Liquids in Graduated Cylinders

Three graduated cylinders. {104vol01.jpg (23000 bytes)} You should be able to measure the volume of liquids in a graduated cylinder. How precisely you can measure volume depends on the size and type of graduated cylinder you use. Generally, you should be able to estimate between the etched or printed lines.
Divisions on 10-mL cylinder. {104vol05.jpg (16504 bytes)} It is important to notice what each line or interval on the graduated cylinder represents. Different kinds of graduated cylinders are set up differently. A 10 milliliter cylinder, for example, usually has one tenth of a milliliter for each graduation, but some have two-tenths milliliter for each graduation. The way to check this is to count the divisions between consecutive numbers. Here we have the usual 10 divisions from one number to the next; therefore, the volume increment for each of those lines is a tenth of a milliliter. Estimating your measurements by reading between the lines, you should be able to measure to a hundredth of a milliliter.
Divisions on 100-mL cylinder. {104vol04.jpg (20376 bytes)} On this 100 milliliter cylinder, the numbers are 10, 20, 30, etc., so there is a 10 milliliter increment between them. Since there are 10 divisions between consecutive numbers, each division represents one milliliter. Therefore, you should be able to estimate to tenths of a milliliter by reading between the lines.
Graduated cylinders with 5 divisions between numbered marks. {104vol06.jpg (23048 bytes)} On some cylinders, there may only be five divisions between numbers. Or there may be ten divisions for a 2 milliliter increment. In these cases, each of the divisions represents 0.2 milliliters, rather than 0.1. You need to be aware of that when you're using the cylinders like these, and adjust your between-line-estimates accordingly.


The Meniscus

Close-up of meniscus. {104vol07.jpg (17576 bytes)} A characteristic of liquids in glass containers is that they curve at the edges. This curvature is called the meniscus. You measure the level at the horizontal center or inside part of the meniscus. With water in glass, the meniscus will curve up at the edges and down in the center so we say you read the bottom of the meniscus.
Mercury meniscus. {104vol15.jpg (19923 bytes)} There are some materials where the curve goes the other way. Still, you read the horizontal center of the meniscus. In this case it would be the top of the meniscus.
Flat meniscus of water in plastic. {104vol14.jpg (12720 bytes)} In some plastic cylinders water has a flat surface. In that case top or bottom doesn't matter, but we can still say use the center rather than the edges.
Meniscus with card behind. {104vol10.jpg (20652 bytes)} Meniscus with card below. {104vol11.jpg (19754 bytes)} The visibility of the meniscus can be enhanced by using a card with a dark stripe on it,  placed behind the cylinder.  Adjusting the placement of the card can give you either a white meniscus against a black background or a black meniscus against a white background.

Now, or when you are in the lab, read through the instructions and complete Exercise 10 in your workbook, and have an instructor check the volume measurements that you make.